The present invention relates to a fixed steel structure for support of a deck having production facilities for oil and gas at large water depths. The invention also relates to a procedure for assembly of the structure while it is floating in the sea.
Fixed trusswork structures of steel for supporting decks with production installations are the most common platform type structures worldwide. The conventional structure contains from four to eight tubular legs which have upper ends above the water surface supporting a rectangular deck and which slope more or less outwards towards the sea bottom where they are fixed by piles. These legs also are interconnected by horizontal and diagonal stays so that the whole structure forms a three dimensional trusswork.
These are concepts which from a building and installation point of view work excellently at small and moderate water depths, i.e. to about 150-200 meters. This type of structure has been used for depths to 300 meters in the Gulf of Mexico.
For larger water depths this type of structure will have great dimensions and weight, and accordingly there will arise considerable drawbacks due to high capacity and production equipment demands at the fabrication yard. For most fabrication yards this will require expansion and new investment, which from a time and economical point of view will be unfavourable. Further, the dimension and weight of such a structure will result in the need for new transportation equipment in the form of larger barges, possibly other transportation methods, and the fabrication installation time will be expensive and the costs will be very high.
An already known proposal of a fixed steel platform for large water depths consists of a central vertical tubular column, the upper part of which below the water surface is connected to three inclined tubular supporting legs which are supported by foundations on the sea bottom. The center column carries a deck with production facilities at a given level above the water surface, and the column contains further risers and other necessary equipment. An arrangement of this type is described in Norwegian patent application No. 830.753; "Offshore construction and method for its arrangement". A substantial problem with platforms at larger depths results from normal oscillations and the appurtenant periods of oscillation of the platform. The highest periods of oscillation should generally be below a level where the excitation from the waves becomes substantial. If the periods of oscillation are too high, the dynamic response will be large and this will especially lead to fatigue problems in the steel material at critical parts of the structure. The periods of oscillation are directed by the stiffness of the structure and the oscillating mass. The stiffness is represented by the geometry of the structure and the quantity of steel in the elements contributing to the stiffness. The mass is represented by the weight of the structure including the deck and its equipment on the platform.
Another substantial contribution to the oscillating mass is the oscillating surrounding water. This oscillating mass of water is proportional to the volume of the structure and it is therefore desirable to reduce this volume to a minimum for those parts of the structure which have largest oscillating movement in the highest periods of oscillation.
The above mentioned proposal of a platform for large water depths is advantageous with regard to the transfer of waveforces and other environmental forces, but the choice of structural elements in the form of steel pipes with great diameter is disadvantageous since regards natural oscillation periods as the structure will have a large volume and the oscillating mass of water will be high. In order to obtain satisfactory periods of oscillation, the amount of steel consequently must be high, i.e. great wall thicknesses of the pipes.
Great wall thicknesses also are necessary with regard to stability and static strength due to the large water pressure acting on the pipes. The weight of the proposed structure for large water depths thus will be very high. Another disadvantage of such structure is the location of risers for gas/oil inside the center column, since the possibility of breaks/leaks therein will be a considerable security problem.